Terahertz (THz) digital holography is an important research aspect of terahertz imaging technology. The recording distance of terahertz digital holography is generally short because of the long wavelength and restriction of the detector. The exact measurement of the real recording distance is of great significance. Autofocusing researches have been made on real 2.52 THz Gabor inline digital holograms based on five kinds of autofocusing criterion functions. The criterion precision is improved by cropping object in the reconstructed images. The experimental results show that the performance of normalized variance criterion function is the best. The results verify the feasibility of applying five kinds of autofocusing criterion functions to terahertz inline digital holography.